Severe acute respiratory syndrome (SARS) is a newly emerging global disease of humans with a major economic impact and significant bioterrorism potential caused by a new strain of coronavirus (CoV). The lung is the target organ related to the disease manifestations, although diarrhea occurs in some patients. Unresolved questions related to SARS pathogenesis include the mechanisms for "superspreaders" and the atypical pneumonia and variable diarrhea induced and the role of polymicrobial infections in the variable severity of SARS. Host immune factors, especially proinflammatory cytokines may play a role in the severe pulmonary damage, as observed in our studies of respiratory disease in pigs. The widespread use of steroids and IFNs for treatment of SARS patients without a clear understanding of their impact on respiratory disease, necessitates studies of their impact in an animal model susceptible to respiratory CoV infection. Although primates are susceptible to SARS CoV, their limited availability and expense hampers comprehensive studies of SARS pathogenesis. In mouse models, the clinicopathological manifestations of CoV or influenza viral infections differ from in humans whereas in pigs they mimic the human disease. The anatomy, physiology and immune system of the pig respiratory tract closely resembles that of man, providing a unique animal model for the study of viral respiratory disease of humans. The porcine respiratory CoV (PRCV), a spike deletion mutant of the enteric CoV transmissible gastroenteritis virus (TGEV), shows striking pathogenetic similarities to SARS CoV in its primary replication in lung. Of interest, PRCV invariably induces similar lung lesions with atypical pneumonia, even in asymptomatic pigs. Our studies suggest that polymicrobial co-infections influence the severity of PRCV infection, lesions and disease via multiple mechanisms. These include the repertoire of proinflammatory cytokines or the cell infiltrates induced in lung, and the multiple cell types infected. Therefore our aim is to determine the influence of steroids and coinfections with respiratory viruses or bacterial derived components (and the cytokines induced) on the severity of a SARS-like respiratory coronavirus (PRCV) infection of swine. Our Specific Aims are: 1) To assess if corticosteroid treatment of PRCV-infected pigs has an impact on cytokines induced by PRCV or acquired immunity to PRCV and the subsequent course of PRCV infection and disease (mimic impact of steroids on SARS patients); 2) To investigate the impact of prior infection with a distantly related (Nidovirales) low pathogenic respiratory viral pathogen (arterivirus, PRRSV) on subsequent PRCV infection and disease (mimic dual SARS CoV and distinct respiratory CoV infections); 3) To explore the impact of initial infection with PRCV followed by subsequent infection with the respiratory viral pathogen swine influenza virus on PRCV infection and disease (mimic dual infections with SARS CoV and influenza); 4) To determine the impact of concurrent infection of pigs with two antigenically related coronaviruses with distinct tissue tropisms (PRCV, respiratory and TGEV, enteric) on generation of PRCV/TGEV recombinants and coronavirus infection and disease (mimic SARS superspeaders with diarrhea); 5) To examine the impact of sequential inoculation of pigs with PRCV followed by bacterial cell wall components on cytokine production and disease (mimic impact of bacterial coinfections on bacterial coinfections on SARS).